1. Field of the Invention
The invention relates to a piezoelectric transformer, and more particularly to a piezoelectric transformer formed at a surface thereof with input and output electrodes. The invention relates also to a support for a piezoelectric transformer.
2. Description of the Prior Art
There has been widely used a coil-wound type electromagnetic transformer as a transforming device for generating a higher voltage to be used for equipments which requires a high voltage, such as a deflector for a cathode ray tube and a charging device for a copier.
On the other hand, a piezoelectric transformer utilizing piezoelectric effect becomes popular for generating a high voltage. FIG. 1A illustrates one of conventional piezoelectric transformers. The illustrated piezoelectric transformer includes a rectangular planar piezoelectric body 131 defining a driven section 135 and a voltage generating section 136 in a direction of a longitudinal axis of the piezoelectric body 131. The piezoelectric body 131 is formed on an upper surface in the driven section 135 with a first electrode 132, and on a lower surface with a second electrode 133 in alignment with the first electrode 132. The first and second electrodes 121 and 133 are electrically insulated with each other, and it is possible to apply a voltage thereacross.
The piezoelectric body 131 is formed at an end surface in the voltage generating section 136 with a third electrode 137. The piezoelectric body 131 including the three electrodes 132, 133 and 137 is polarized in a widthwise direction thereof in the driven section as indicated with an arrow X1, and polarized in a lengthwise direction thereof in the voltage generating section 136 as indicated with an arrow X2. The piezoelectric transformer is supported and fixed on a support 139 at the center in a direction of a longitudinal axis thereof.
In order to boost a voltage by means of the illustrated piezoelectric transformer, an ac voltage Ein is first applied across the first and second electrodes 132 and 133. The ac voltage Ein is selected to have a frequency equal to a resonance frequency of a longitudinal oscillation of the piezoelectric body 131. By applying the thus selected ac voltage across the first and second electrodes 132 and 133, the piezoelectric body 131 is mechanically resonated, resulting in that a voltage Eout which is higher than Ein and has the same frequency as that of Ein is generated at the voltage generating section 136. The thus generated higher voltage Eout can be obtained between the second (or first) electrode 133 and the third electrode 137. The support 139 supports the piezoelectric transformer at a nodal point of longitudinal oscillation of the piezoelectric body 131.
The above mentioned operation concerns only voltage step-up. By substituting the first and second electrodes 132 and 133 used as input ports and the third electrode 137 used as an output port with each other, that is, by applying an ac voltage across the second and third electrodes 133 and 137 and obtaining a voltage between the first and second electrodes 132 and 133, the illustrated piezoelectric transformer acts also as a step-down transformer. It depends on the purpose of using the piezoelectric transformer whether electrodes formed on a surface of a piezoelectric body are used as input or output electrodes, and whether sections defined in a piezoelectric body are used as driven or voltage generating sections.
In order to put the above mentioned piezoelectric transformer illustrated in FIG. 1A to practical use, it is necessary to provide the first to third electrodes 132, 133 and 137 with a terminal such as a lead wire for electrically connecting the piezoelectric transformer to an external circuit. However, such lead wires are not always connected to the electrodes 132, 133 and 137 both in the driven and voltage generating sections 135 and 136 at nodal points of mechanical oscillation of the piezoelectric body 131. Thus, the lead wires connected to the electrodes are often broken down. In order to enhance reliability in connection between the surface electrodes and the lead wires, it is effective that lead wires are connected to surface electrodes at nodal points of oscillation of a piezoelectric body. To this end, there are often defined three sections in a piezoelectric body.
FIG. 1B illustrated an example of a piezoelectric transformer having a piezoelectric body 141 including three sections defined therein in a direction of a longitudinal axis thereof. Namely, the piezoelectric body 141 includes a left side driven section 145L, a right side driven section 145R, and a voltage generating section sandwiched the driven sections 145L and 145R. The voltage generating section is partitioned into two sections 146L and 146R by a strip-shaped electrode 147 wound around the piezoelectric body 141 at a longitudinal midpoint of the piezoelectric body 141. Thus, the piezoelectric body 141 includes four sections: the left side driven section 145L, left side voltage generating section 146L, right side voltage generating section 146R and right side driven section 145R, which sections are arranged symmetrically about the electrode 147. The piezoelectric body 141 is formed on upper and lower surfaces thereof in the left side driven section 145L with input electrodes 142L and 143L. Similarly, the piezoelectric body 141 is formed on upper and lower surfaces thereof in the right side driven section 145R with input electrodes 142R and 143R. The driven sections 143L and 143R are polarized in a thicknesswise direction of the piezoelectric body 141, as indicated with an upwardly directed arrow X1. The voltage generating sections 146L and 146R are polarized in a lengthwise direction of the piezoelectric body 141, similarly to the piezoelectric transformer illustrated in FIG. 1A, but in opposite directions, as indicated with arrows X2 and X3. That is, the left side voltage generating section 146L is polarized to the left in a direction of a longitudinal axis of the piezoelectric body 141, as indicated with the arrow X2, whereas the right side voltage generating section 146R is polarized to the right in a direction of a longitudinal axis of the piezoelectric body 141, as indicated with the arrow X3. The upper input electrodes 142L and 142R are electrically connected by a wire W1, and the lower input electrodes 143L and 143R are electrically connected by a wire W2. An ac voltage Ein is applied across the upper input electrodes 142L and 142R and the lower input electrodes 143L and 143R through the wires W1 and W2. An output voltage Eout, which is higher than Ein, is obtained between the electrode 147 and the upper input electrodes 142L and 142R. The ac voltage Ein is selected to have a frequency which causes tertiary resonance in a direction of a longitudinal axis of the piezoelectric body 141. By applying the thus selected ac voltage Ein to the electrodes, the piezoelectric body 141 has three nodal points in oscillation thereof: a point A1 at a distance of one-sixth of a full longitudinal length L of the piezoelectric body 141 away from an end surface 14a of the piezoelectric body 141; a point A2 at a distance of one-sixth of the length L away from the other end surface 141b of the piezoelectric body 141; a point A3 located at the midpoint of the length L. Thus, it is possible to prevent break-down of lead wires due to oscillation of the piezoelectric body 141 by connecting the lead wires to the surface electrodes 142L, 143L, 142R, 143R and 147 at the nodal points A1, A2 and A3.
The planar piezoelectric transformers as mentioned above is small in size, but can produce a high voltage, and hence is very attractive, for instance, to an invertor to be used for a liquid crystal back light, which is requested to be smaller in size. However, the planar piezoelectric transformers still have the following problems to be solved which are accompanied by the fact that a piezoelectric transformer is planar.
The first problem is as follows. When a piezoelectric body is oscillated, a larger internal stress is generated in the vicinity of nodal points of oscillation in the piezoelectric body. Specifically, tensile stresses are produced at a boundary between the driven section 135 and the voltage generating section 136 in the piezoelectric transformer illustrated in FIG. 1A and in the vicinity of the electrode 147 in the piezoelectric transformer illustrated in FIG. 1B, respectively, and thus the piezoelectric bodies 131 and 141 are in conditions to be readily destructive. A piezoelectric transformer is usually designed so that the tensile stress is lower than a critical value for destruction of a piezoelectric body. However, ceramic of which a piezoelectric body is often made is likely to be chipped even by small impact, and a chipping has occurred once in a piezoelectric body, the critical value is significantly lowered, resulting in that a piezoelectric body is easily destroyed.
A chipping as mentioned above in a piezoelectric body is unavoidable in piezoelectric body fabrication steps and subsequent steps, and hence is a major factor for preventing enhancement in reliability to a piezoelectric transformer. In addition, a piezoelectric body has to be treated with the greatest possible care, which increases fabrication costs of a piezoelectric transformer. A chipping, reduction in reliability due to a chipping, and increased fabrication costs as mentioned above are problems in particular when a piezoelectric body is formed in a rectangular parallelopiped such as a plate and a prism. This is because a rectangular parallelopiped has many edges at which a chipping is prone to take place.
The second problem of a planar piezoelectric transformer is that noises tend to be produced in audible ranges. Since a piezoelectric transformer utilizes mechanical oscillation, a frequency for operation of a piezoelectric transformer is selected to be in the range of about 30 to about 150 kHz which is beyond audible range. In a planar piezoelectric transformer, there would be generated longitudinal oscillation in a widthwise direction, bending oscillation and torsional oscillation as well as longitudinal oscillation in a direction of a longitudinal axis of a piezoelectric body. Thus, frequencies of those oscillations, a beat frequency between two different oscillations, and a beat frequency between higher mode frequency of various oscillations and another mode frequency enter audible range. Such frequencies entering audible range would be merely a noise to men's ears, which poses a problem in putting a planar piezoelectric transformer into practical use. Even if a piezoelectric transformer is fabricated in the form other than a plate, there would be produced various oscillation modes. However, a piezoelectric transformer fabricated in the form of a planar plate would quite readily produce unpreferable modes of oscillation, which oscillation in addition is likely to become greater in level.
In order to solve the above mentioned problems caused by the fact that a piezoelectric body is planar in shape, there has been suggested piezoelectric bodies in various forms.
U.S. Pat. No. 2,974,296 has suggested a columnar piezoelectric transformer. The piezoelectric transformer has a thin-walled columnar piezoelectric body defining three sections in a direction of a longitudinal axis thereof. Ring-shaped electrodes are wound around the piezoelectric body at boundaries among the three sections and at opposite ends. Among the four electrodes disposed longitudinally of the piezoelectric body, two of the internally located electrodes are used as driven electrodes, whereas two of the externally located electrodes are used as voltage generating electrodes. Since it is difficult in the above mentioned piezoelectric transformer to make a step-up ratio greater, which is defined as a ratio of an input voltage to an output voltage, U.S. Pat. No. 2,974,296 has also suggested a piezoelectric transformer including a driven section defined between outer and inner surfaces of a piezoelectric body, and a voltage generating section defined over a full length of a piezoelectric body.
Japanese Unexamined Patent Publication No. 2-311181 has suggested a columnar piezoelectric motor including cross type finger electrodes.
Japanese Unexamined Patent Publication No. 2-163982 has suggested a piezoelectric actuator including electrodes spirally extending on an outer surface thereof.
Japanese Unexamined Patent Publication No. 4-307322 has suggested a columnar piezoelectric gyro.
The piezoelectric transformer suggested in U.S. Pat. No. 2,974,296 does not have purposes of prevention of chipping in a piezoelectric body and prevention of noises. The above mentioned piezoelectric motor, piezoelectric actuator and piezoelectric gyro are devices used for electrical-mechanical energy conversion or mechanical-electrical energy conversion. In a device such as a motor or a gyro necessarily including mechanical rotation, a piezoelectric body is inevitably columnar in shape. In contrast, a piezoelectric transformer acting as an electrical-mechanical energy convertor may have any shape, but it is not known so far to fabricate a piezoelectric transformer including a cylindrical piezoelectric body.